Stay Comfy Blog

Furnace Flame Sensor 101: What You Need to Know

We all know how important our furnace is — especially in our Upper Midwest winters. Imagine losing heat on one of our sub-zero temp days! At the same time, many homeowners aren’t familiar with the different components of their furnace and just how important some of them are. That’s especially true of a furnace’s flame sensor, which is a very important safety feature. Here's everything you need to know about your flame sensor and how to keep it in tip-top shape.

What is a Furnace Flame Sensor?

The flame sensor in your furnace is a safety device that ensures the gas that fuels your furnace is being burned and not escaping into the vent system. The sensor detects when a flame is burning inside the burner assembly of your furnace. If no flame is detected, the sensor shuts off the furnace to avoid unburned gas from being lost into the vent system or chimney. In older furnaces (those without draft inducers) it could cause a build up unburned fuel and cause a “hard ignition” (sort of a minor explosion) that could do damage to the furnace.

How a Flame Sensor Works

The sensor itself is a small, metallic rod with a porcelain base that conducts an electrical current. When the gas valve opens at the start of the heating process, that electrical current detects whether or not a flame is present by sensing heat from the flame. If that’s the case, then the gas ignites and the heating cycle continues. If, however, the flame sensor does not detect a flame — usually within 10 seconds — it will shut the furnace down.

Typically, the cycle will repeat one or two more times, but if it still doesn’t detect a flame, your furnace may be “locked-out” and require a reset. Most furnaces can be reset by turning off the power momentarily, or by turning the furnace off at the thermostat, then wait a few seconds and then turn it back on.

Signs Your Flame Sensor May be Failing

As with any HVAC component, catching a problem before it occurs is always a plus. Here are three tell-tale signs that your sensor may be going bad:

Your gas burners light but go out almost immediately.

There is visible soot, dust, or rust that covers the tip of the flame sensor itself.

The porcelain area of the sensor unit is cracked, chipped, or broken.

Cleaning Your Flame Sensor

If you suspect your flame sensor is going bad — and you’re the intrepid, DIY sort — you can inspect and clean it yourself, following these steps.

Turn off your furnace completely.

Open the combustion chamber door to gain access to the sensor.

Remove the sensor and clean it using steel wool, light-grit sandpaper, or emery cloth. Be sure to remove any dust, grit, or rust.

Clear any dust from the furnace pilot and electronic igniter, then replace the sensor, and close the combustion chamber door.

Restart your furnace.

When to Call in the Pros

Of course, not every homeowner wants to take apart their furnace. Plus, there are times when even those who do are better off calling in an HVAC professional. If you’ve cleaned your sensor and it’s still giving you trouble, or if you’d rather not tackle the job yourself, call your HVAC professional as soon as you sense a potential problem. You’ll be glad you did — it will ensure the safety and comfort of you and your family.